The present invention relates to the measurement of fuel flow and particularly to the measurement of fuel flow in aircraft engines.
In aircraft engines, a mass flow measurement device (flowmeter) is usually positioned immediately downstream of an engine fuel control unit in order to measure the engine consumed fuel mass flow rate. The fuel flowmeter typically contains a driver assembly and a measurement assembly. The driver assembly includes a turbine at the unit inlet, the flow of the fuel causing the turbine to rotate in a clockwise direction. The flowmeter is designed such that the turbine rotates at a generally constant speed independent of mass fuel flow.
Once the fuel has passed through the turbine, it enters a fixed straightener section where the swirl and flow aberrations are removed. When the fuel exits the straightener, it is therefore substantially straight and axial.
The fuel then enters the measurement assembly which is caused to rotate by the action of the turbine. The measurement assembly includes a shaft to which an enclosing drum is rigidly attached. The shaft is rotated by the turbine. The measurement assembly also includes an impeller which is located within the drum and is mounted for rotation on the shaft via precision bearings. The impeller is connected to the drum by a hairspring which resists relative rotational movement between the impeller and the drum, about the shaft.
As the mass of fuel flows into the rotating measurement assembly, it causes the hairspring mounted impeller to rotate (relative to the drum) as the fuel is caused to rotate. The angular displacement between the drum and the impeller is measured by two pick-off coils, one adjacent to the drum and the other adjacent to the impeller. As permanent magnets fixed to the drum and to the impeller rotate past the associated stationary pick-off coils, pulses are induced in each coil. The time (t) between the respective pulses is a measure of mass fuel flow. Calibration is carried out under laminar flow conditions.
The above flowmeter suffers from inaccuracies, particularly at lower fuel flows, which includes cruise conditions where accuracy is particularly important. The errors are not systematic and may be higher or lower than the true reading.
According to the invention there is provided apparatus for metering fuel mass flow, the apparatus including:
a rotatable member through which fuel to be metered may pass;
a gauge associated with the rotatable member, for providing a measure of mass flow rate of fuel therethrough;
means for rotating the rotatable member as fuel is passed therethrough; and
means for measuring the rate of rotation of the rotatable member and applying a correction factor to the measured mass flow rate, the magnitude of the correction factor depending upon the measured rate of rotation of the rotatable member.
Preferably the apparatus includes means for comparing the measured rate of rotation of the rotatable member with a nominal rate of rotation.
Preferably the apparatus includes means for rotating the rotatable member about an axis which is substantially parallel to the direction of movement of fuel through the rotatable member.
Preferably the gauge comprises a vane member rotatable with the rotatable member and capable of rotation relative to the rotatable member, about the axis, but biased against such relative rotation.
Preferably the vane member is freely mounted for rotation about the axis but is attached to the rotatable member via a spring which provides the bias.
The vane member may include a plurality of radially extending vanes, the vanes resisting rotational movement of the vane member relative to the fuel flowing through the rotatable member.
Preferably the relative rotational positions of the vane member and the rotatable member are representative of the mass fuel flow through the rotatable member.
Preferably the apparatus further includes a magnet mounted on the rotatable member and a stationary detector mounted adjacent the rotatable member, the detector providing an indication of each passage of the magnet and therefore of the rate of rotation of the rotatable member.
Preferably the apparatus further includes a magnet mounted on the vane member and a stationary detector mounted adjacent the vane member, the detector providing an indication of each passage of the vane member and therefore of the rate of rotation of the vane member.
The apparatus may further include means for comparing the signals from the respective detectors to provide an indication of the phase of the rotation of the vane member relative to the rotation of the rotational member and therefore of their relative angular positions.
Preferably the means for rotating the rotatable member includes a turbine comprising a plurality of aerofoils.
According to the invention there is further provided a method for metering fuel mass flow, the method including the steps of:
providing a rotatable member through which fuel to be metered may pass;
providing a gauge associated with the rotatable member, for providing an indication of mass flow rate of fuel therethrough;
rotating the rotatable member as fuel is passed therethrough; and
measuring the rate of rotation of the rotatable member and applying a correction factor to the measured mass flow rate, the magnitude of the correction factor depending upon the rate of rotation of the rotatable member.
Preferably the method includes the step of comparing the measured rate of rotation with a known nominal rate of rotation of the rotatable member.
Preferably the correction factor also depends upon the measured mass fuel flow.
The correction factor may have been predetermined experimentally.